Drosophila melanogaster

Drosophila melanogaster (Black-bellied Dew-lover) a dipteran (two-winged) insect, is the species of fruit fly that is commonly used in genetic experiments; it is among the most important model organisms. In modern biological literature, it is often simply called Drosophila or (common) fruit fly.

Physical appearance

The flies have red eyes, a yellow-brown color, with transversal black rings across their abdomen. Females are about 2.5 millimetres long; males are slightly smaller and the back of their bodies is darker. For a neophyte trying to tell the difference between the sexes under a dissecting microscope, perhaps the easiest distinguishing mark is the cluster of spiky hairs surrounding the anus and genitals of the male. There are extensive images at flybase (see link below).

Life cycle

The life cycle of Drosophila melanogaster at 25 °C takes only 2 weeks; everything takes about twice as long at 18 °C. Females lay some 400 eggs (embryos) into rotting fruit or other organic material. The eggs, which are about 0.5 millimetres long, eclose after 24 h. The resulting larvae grow for 5 days while molting twice, at about 24 and 48 h after eclosion. During this time, they feed on the microorganisms that decompose the fruit, as well as on the sugar of the fruit themselves. Then the larvae encapsulate in the puparium and undergo a five-day-long metamorphosis, after which the adults emerge.

Females first mate about 12 hours after emergence. The females store sperm from previous males they mated with for later use. For this reason geneticists must collect the female fly before her first mating, that is, a virgin female, and ensure that she mates only with the particular male needed for the experiment. Inseminated females can be "re-virginized" by prolonged incubation at -10 °C, which kills the sperm, according to Michael Ashburner's "red book".

Model organism in genetics

Drosophila melanogaster is one of the most studied organisms in biological research, particularly genetics and developmental biology. There are several reasons:

It is small and easy to grow in the laboratory
It has a short generation time (about 2 weeks) and high productivity (females can lay 500 eggs in 10 days)
The mature larvae show giant chromosomes in the salivary glands.
It has only 4 pairs of chromosomes: 3 autosomal, and 1 sex.
Males do not show recombination, facilitating genetic studies.
Genetic transformation techniques have been available since 1987.
Its compact genome was sequenced in 1998.

Charles W. Woodworth is credited with being the first to breed Drosophila in quantity and for suggesting to W. E. Castle that they might be used for genetic research during his time at Harvard University. Beginning in 1910, fruit flies helped Thomas Hunt Morgan accomplish his studies on heredity. "Thomas Hunt Morgan and colleagues extended Mendel's work by describing X-linked inheritance and by showing that genes located on the same chromosome do not show independent assortment. Studies of X-linked traits helped confirm that genes are found on chromosomes, while stuies of linked traits led to the first maps showing the locations of genetic loci on chromosomes" (Freman 214). The first maps of Drosophila chromosomes were completed by Alfred Sturtevant.

The Drosophila genome

The genome of Drosophila contains 4 pairs of chromosomes: an X/Y pair, and three autosomes labeled 2, 3, and 4. The fourth chromosome is so tiny that it is often ignored. The genome contains about 165 million bases and approximately 14,000 genes. The genome has been sequenced and has been annotated.

Similarity to humans

Genetically speaking, people and fruit flies are very similar. About 61% of known human disease genes have a recognizable match in the genetic code of fruit flies, and 50% of fly protein sequences have mammalian analogues. Drosophila is being used as a genetic model for several human diseases including Parkinson's and Huntington's.

Genetic nomenclature

Genes named after recessive alleles begin with a lowercase letter, while dominant alleles begin with a uppercase letter. Genes named after a protein product begin with an uppercase letter. Genes are typically written in italics.
The convention for writing out genotypes is X/Y; 2nd/2nd; 3rd/3rd.²

In the molecular biology community, Drosophila geneticists are known for their relatively whimsical naming of discovered gene mutations. Compared to the stodgy (but perhaps more practical) "cdc4", "cdk4", etc. names in the yeast genome, Drosophila sports such favorites as "cheap date" (a mutation leading to increased sensitivity to ethanol intoxication) and "snafu" (a mutation leading to grotesque anatomical abnormalities).

Development and embryogenesis

Main article: Drosophila embryogenesis

Embryogenesis in Drosophila has been extensively studied, the small size, short generation time, and large brood size makes it ideal for genetic studies. It is also unique among model organisms in that cleavage occurs in a syncytium.

During oogenesis, cytoplasmic bridges connect the forming oocyte to nurse cells. Nutrients and developmental control molecules move from the nurse cells into the oocyte. In the figure to the left, the forming oocyte can be seen to be covered by follicular support cells.

About 5000 nuclei accumulate in the unseparated cytoplasm of the oocyte before they migrate to the surface and are encompassed by plasma membranes to form cells surrounding the yolk sac. Early on, the germ line segregates from the somatic cells through the formation of pole cells at the posterior end of the embryo.

Behavioral genetics and neuroscience

In 1971 Ron Konopka and Seymour Benzer published a paper titled "Clock mutants of Drosophila melanogaster" in which they described the first mutations that affected an animal's behavior. Wild-type flies show an activity rhythm of with a frquency of about a day (24 hours). They found mutants with faster and slower rhythms as well as broken rhythms - flies that move and rest in random spurts. Work over the next 30 years has shown that these mutations (and others like them) affect a group of genes and their products that comprise a biochemical or molecular clock. This clock is found in a wide range of fly cells, but the clock-bearing cells that control activity are several dozen neurons in the fly's central brain.

Since then Benzer, his students and many other have used behavioral screens to isolate genes involved in vision, olfaction, audition, learning/memory, courtship, pain and other processes such as longevity.

Vision in Drosophila

The compound eye of the fruit fly contains 800 unit eyes or ommatidia. Each ommatidium contains 8 photoreceptor cells (R1-8), support cells, pigment cells, and a cornea. Wild-type flies have reddish pigment cells, which serve to absorb excess blue light so the fly isn't blinded by ambient light.

Each photoreceptor cell consists of two main sections, the cell body and the rhabdomere. The cell body contains the nucleus while the rhabdomere is made up of toothbrush-like stacks of membrane called microvilli. Each microvillus is 1 mm to 1.5 mm in length and 50 nm in diameter. The membrane of the rhabdomere is packed with about 100 million rhodopsin molecules, the visual protein that absorbs light. The rest of the visual proteins are also tightly packed into the microvillar space, leaving little room for cytoplasm.

The photoreceptors in Drosophila express a variety of rhodopsin isoforms. The R1-R6 photoreceptor cells express Rhodopsin1 (Rh1) which has absorbs blue light (480 nm). The R7 and R8 cells express a combination of either Rh3 or Rh4 which absorb UV light (345 nm and 375 nm), and Rh5 or Rh6 which absorb blue (437 nm) and green (508 nm) light respectively. Each rhodopsin molecule consists of an opsin protein covalently linked to a carotenoid chromophore, 11-cis-3-hydroxyretinal.³

As in vertebrate vision, visual transduction in invertebrates occurs via a G protein-coupled pathway. However, in vertebrates the G protein is transducin, while the G protein in invertebrates is Gq (dgq in Drosophila). When rhodopsin (Rh) absorbs a photon of light its chromophore, 11-cis-3-hydroxyretinal, is isomerized to all-trans-3-hydroxyretinal. Rh undergoes a conformational change into its active form, metarhodopsin. Metarhodopsin activates Gq, which in turn activates a phospholipase Cβ (PLCβ) known as NorpA.

PLCβ hydrolyzes phosphoinositol-4,5-bisphosphate (PIP₂), a phospholipid found in the cell membrane, into soluble inositol triphosphate (IP₃) and diacylgycerol (DAG), which stays in the cell membrane. DAG or a derivitive of DAG causes a calcium selective ion channel known as TRP (transient receptor potential) to open and calcium and sodium flows into the cell. IP₃ is thought to bind to IP₃ receptors in the subrhabdomeric cisternae, an extension of the endoplasmic reticulum, and cause release of calcium, but this process doesn't seem to be essential for normal vision.⁴

Calcium binds to proteins such as calmodulin (CaM) and an eye-specific protein kinase C (PKC) known as InaC. These proteins interact with other proteins and have been shown to be necessary for shut off of the light response. In addition, proteins called arrestins bind metarhodopsin and prevent it from activating more Gq.

A potassium-dependent sodium/calcium exchanger known as NCKX30C pumps the calcium out of the cell. It uses the inward sodium gradient and the outward potassium gradient to extrude calcium at a stoichiometry of 4 Na⁺/ 1 Ca⁺⁺, 1 K⁺.⁵

TRP, InaC, and PLC form a signaling complex by binding a scaffolding protein called InaD. InaD contains five binding domains called PDZ domains which specifically bind the C termini of target proteins. Disruption of the complex by mutations in either the PDZ domains or the target proteins reduces the efficiency of signaling. For example, disruption of the interaction between InaC, the protein kinase C, and InaD results in a delay in inactivation of the light response.

Unlike vertebrate metarhodopsin, invertebrate metarhodopsin can be converted back into rhodopsin by absorbing a photon of orange light (580 nm).

Approximately two-thirds of the Drosophila brain (about 200,000 neurons total) is dedicated to visual processing. Although the spatial resolution of their vision is significantly worse than that of humans, their temporal resolution is approximately ten times better.

Drosophila flight

The wings of a fly are capable of beating at up to 250 times per second. Flies fly via straight sequences of movement interspersed by rapid turns called saccades. During these turns, a fly is able to rotate 90 degrees in less than 50 milliseconds.

It was long thought that the characteristics of Drosophila flight were dominated by the viscosity of the air, rather than the inertia of the fly body. However, recent research by Michael Dickinson and Rosalyn Sayaman has indicated that flies perform banked turns, where the fly accelerates, slows down while turning, and accelerates again at the end of the turn. This indicates that inertia is the dominant force, as is the case with larger flying animals.